I am a SQL Server guy having 10+ years of experience, but very new to Oracle. I have been asked to do some R&D on data security in oracle. We have a very huge data on Oracle 11gR2. We are planning to move to cloud environment.
Since data is very critical and confidential, we wanted to secure it at any cost without any performance issues with the application. We cloud DBAs also should not be able to see the data.

Can any expert suggest procedure to follow.
I have already done some R&D on Transparent Data Encryption at Tablespace level. I don't think it is sufficient to protect the data?

we wanted to secure it at any cost without any performance issues with the application. We cloud DBAs also should not be able to see the data.

"At any cost"?
I can almost guarantee management will change that when they see the actual cost of what they are asking for.

And as Michel Cadot said, clouding and security are mutually exclusive. As we used to say when I worked for an organization with high security requirements "Ultimately, the guy who can put his finger on the server's power button owns your data."